The Queen's birthday is officially celebrated every June by the ceremony of Trooping the Colour on Horse Guards Parade.
Although she was born on the 21st of April, it has long been customary to celebrate her birthday publicly on a day in the summer, when better weather is more likely.
The ceremony dates back to the early 18th century or earlier, when the colours (flags) of the battalion were carried (or 'trooped') down the ranks so that they could be seen and recognised by the soldiers.
The Queen has taken the salute every year of her reign, except in 1955 when a national rail strike cancelled the event.
!Note - you could also write "on 21 April" or "on 21st April".

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